1. Field of the Invention
This invention relates to an upgrading method of low-rank coal and, particularly, relates to a method for upgrading low-rank coal having higher ash and moisture contents to provide coal having a heightened heating value.
2. Description of the Prior Art
Most of coals which have heretofore been used widely as fuel or the like are high-rank coals such as bituminous coal. On the other hand, although low-rank coal accounts for about one fourth of the total of coal existing on the earth, it has not yet been utilized fully and effectively because of its low heating value as fuel; that is, it has a low heating value because of its higher ash and moisture contents and, when burned in a boiler or the like, a phenomenon of marked abrasion on the boiler tubes occurs. Accordingly, in order to provide effective utilization of such low-rank coal as fuel or the like, it becomes important to decrease the ash and moisture contents and thereby to upgrade the coal into coal having a greater heating value.
As a method of decreasing the ash content of coal, there is a conventional coal preparation method. This method, however, has a drawback that the recovery ratio of coal is low. Accordingly, an oil agglomeration method has recently been proposed which can decrease the ash content of coal and, at the same time, provide a markedly high recovery ratio of the coal. This method comprises the steps of pulverizing coal, slurrying the pulverized coal in a liquid medium such as water, then adding a binder for coal to this slurry and mixing the slurry under agitation to effect flocculation and agglomeration of the coal to form agglomerates. In such a process, ash which is hydrophilic is separated from coal in the flocculation and agglomeration steps and, consequently, the ash content of coal can be decreased and, in addition, markedly high recovery ratio of coal can be achieved because the coal is recovered in the form of agglomerates. Accordingly, the inventors of this invention have conducted such a method on low-rank coal having an ash content of more than 20% (based on wet coal) and a moisture content of more than 20% (based on wet coal). The term "wet coal" means the as received coal from a mine, therefore, wet coal contains the surface moisture (or free moisture) and inherent moisture (or equilibrium moisture). So the term "based on wet coal" means "as received basis." As a result, it has been found that, in case of such low-rank coal, no agglomerates can be formed and, further, separation of ash from coal does not occur as is quite different from the case of high-rank coal.